Light emitting diodes (LEDs) are rapidly replacing incandescent and fluorescent light sources in many illumination systems. LEDs emit light in the ultraviolet, visible and infrared regions of the optical spectrum. Gallium nitride (GaN) based LEDs, for example, emit light in the ultraviolet, blue, cyan and green spectral regions. AlGaInP LEDs emit light in the yellow and red regions of the optical spectrum.
Some illumination applications require a thin, low profile structure. For example, a backlight for a liquid crystal display (LCD) on a laptop computer or desktop computer monitor presently uses one or more thin cold cathode fluorescent lamps (CCFLs) that are coupled into a thin transparent optical waveguide. The waveguide is a solid plastic sheet that has surface features, such as grooves or roughened areas or white painted spots, which scatter light out of the waveguide to form a thin uniform source of light. The light exiting the backlight is directed predominately perpendicular to the plane of the waveguide. The light emitted by the thin planar waveguide is directed through the LCD panel to the person viewing the display. For relatively small displays, one can replace the CCFL light source with an array of LEDs that are positioned along the edges of the waveguide.
Larger displays, in particular LCD television displays, require a large area backlight. As the backlight become larger, it is no longer convenient to place LED light sources along the edges of the waveguide. When the LEDs are placed only along the edges of the waveguide, the edges of the display may be brighter than the center of the display, which is undesirable. In order to have a uniformly bright LED-based backlight, the LEDs must be embedded within holes scattered across the area of the waveguide. At each hole in the waveguide structure, a waveguide input surface surrounds an LED. A side emitting LED structure is desirable for these types of applications in order to direct the LED light into the input surface of the waveguide. Once the light is inside the waveguide, additional turning elements within the waveguide structure can redirect the light perpendicular to the plane of the waveguide and through the LCD panel.
For a very large LCD television such as a 37-inch or larger diagonal display, the solid plastic waveguide becomes very heavy and expensive. In addition, the plastic material such as acrylic that is used for the waveguide absorbs a considerable amount of blue light. For these very large displays, it is desirable to get rid of the plastic waveguide altogether and use a reflecting box that contains the light sources and is filled with air. However, the air filled box still needs to be thin. If LEDs are utilized as the light source, it is preferred that the LED structures be side emitting LED structures in order to spread the emitted light over a large area of the reflecting box and to prevent bright spots in the portions of the display directly in front of the LEDs. Light from the side emitting LED structures is redirected through the LCD panel by turning elements such as diffuse reflecting surfaces or angled reflectors.
An important parameter to consider in the design of LED-based LCD backlights is the reflectivity of the LEDs to externally incident light. Many commercially available LEDs, including the GaN-based LEDs made from GaN, InGaN, AlGaN and AlInGaN, have relatively low reflectivity to externally incident light. One reason for the low reflectivity is the high optical absorption of the LED semiconductor layers at the emitting wavelength of the internally generated light. Due to problems fabricating thin layers of the semiconductor materials, an absorption coefficient greater than 50 cm−1 is typical.
Another reason for the low reflectivity of many present LED designs is that the LED die may include a substrate that absorbs a significant amount of light. For example, GaN-based LEDs with a silicon carbide substrate are usually poor light reflectors with an overall reflectivity of less than 40%.
An additional reason for the low reflectivity of many present LED designs is that the external structures on the LEDs, including the top metal electrodes, metal wire bonds and sub-mounts to which the LEDs are attached, are not designed for high reflectivity. For example, the top metal electrodes and wire bonds on many LEDs contain materials such as gold that have relatively poor reflectivity for light wavelengths less than about 550 nanometers. Reflectivity numbers on the order of 35% in the blue region of the optical spectrum are common for gold electrodes.
Due to the low reflectivity (less than 40%, for example) of many commercially available LEDs, illumination systems that incorporate such LEDs are designed to allow little or no light to return to the LEDs. Any light that is directed toward a poorly reflecting LED may be absorbed and lower the overall efficiency of the illumination system.
Some types of LEDs exist that have relatively high reflectivity, but such LEDs generally have low light extraction efficiency (for example, less than 25%). Illumination systems designed with such LEDs have low overall efficiency due to the low extraction efficiency of light from the LED structure.
It is possible to construct LEDs that have both high reflectivity to externally incident light and high light extraction efficiency. Examples of highly reflective, high efficiency LEDs are disclosed by Beeson and Zimmerman in U.S. patent application Ser. No. 10/952,112 entitled “LIGHT EMITTING DIODES EXHIBITING BOTH HIGH REFLECTIVITY AND HIGH LIGHT EXTRACTION” and in U.S. patent application Ser. No. 11/185,996 entitled “LIGHT EMITTING DIODES WITH IMPROVED LIGHT EXTRACTION AND REFLECTIVITY,” both of which are herein incorporated by reference. LEDs are disclosed that do not require a large transparent optical element such as a hemispherical lens in order to achieve relatively high light extraction. Using such LEDs can allow illumination systems to be designed such that light is recycled back to the LED structures and is reflected by the LED structures. Light that is reflected by the LED sources will increase the effective brightness of the LED sources and increase the output brightness and efficiency of the illumination system. If both the LED reflectivity to externally incident light and the light extraction efficiency of the LED are high, a high efficiency, light recycling illumination system can be constructed.
LEDs with side emitting lenses are disclosed in U.S. Pat. No. 6,679,621. A complex lens having a curved reflective surface and curved and oblique angled refracting surfaces will reflect and refract light from an LED at an approximately right angle. However, the typical height of the side-emitting complex lens is 6 mm or larger. This relatively large size prevents the use of the side emitting lens in, for example, ultra-thin liquid crystal display (LCD) backlight structures that are thinner than about 6 mm. In order to produce ultra-thin illumination systems, it would be desirable to shorten or eliminate the lens but still retain high light extraction efficiency. U.S. Pat. No. 6,679,621 does not disclose low profile illumination systems that are thinner than about 6 mm and does not disclose recycling of emitted light back to the LEDs in order to increase the effective brightness of the LEDs and to increase the output brightness and efficiency of the illumination system.
Low profile illumination systems incorporating LEDs are disclosed in U.S. Pat. No. 6,473,554. Light exits the LED into a cusp-shaped reflector, is reflected approximately at right angles and then exits the reflector approximately parallel to the output surface of the LED. U.S. Pat. No. 6,473,554 does not disclose recycling of emitted light back to the LEDs in order to increase the effective brightness of the LEDs and to increase the output brightness and efficiency of the illumination system.
It would be desirable to develop side-emitting LED-based illumination systems that have a thin profile and that allow for light to be recycled back to the LED sources in order to increase the effective brightness of the LED sources and to increase the output brightness and efficiency of the illumination systems. For side-emitting illumination systems that incorporate LEDs having multiple colors or that incorporate wavelength conversion materials such as phosphors, it would be desirable to utilize light recycling in order to improve color mixing and to improve the color uniformity of the output light. Such side-emitting illumination systems can be used in applications such as LCD backlights that require a high-brightness, low profile illumination source.